The present invention relates to a copper fin material for a heat-exchanger, suitable for use under the severe conditions and corrosive environment of automobile engines, etc. The present invention also relates to a method of producing the same. In particular, the present invention has made it possible to improve the corrosion resistance of a copper fin material and to thin the fin without decreasing its thermal conductivity.
Recently, a trend in the lightening in weight of automobile radiators has been associated with thinning the fin material for heat-exchangers. On the other hand, the corrosion due to the salt damage caused by snow-melting material etc. has become a problem. The severe corrosion of fin material due to salt damage seriously affects the heat-exchanger, such as decreasing the radiating characteristics, deteriorating the strength and the like.
In general, improvements in the strength, corrosion resistance, etc. are all desired for the heat-exchanger fin material. With respect to the improvement in corrosion resistance, the improvement is possible by alloying the material through the addition of second and third elements such as, for example, Cu-Ni type anticorrosive alloys. This brings about, however, not only an increase in cost resulting in an economical disadvantage, but also a drastic decrease in thermal conductivity (electroconductivity). Hence, even if the fin material exhibits excellent corrosion resistance, it ends up becoming quite unsuitable as a heat-exchanger fin material, high electroconductivity being desired therefor.
On the other hand, corrosion is principally a phenomenon on the surface. Thus, if only the surface of the material is modified, it should be possible to substantially maintain the electroconductivity, and yet, improve the corrosion resistance. Based on this thought, a fin material suitable for a car radiator has been proposed, wherein the fin material has a diffused layer of Zn formed on the surface of a highly electroconductive copper-based material. Thus, the inside core material is protected by a sacrificial anode effect, yet the electroconductivity of the core material is retained. In fact, a distinct effect on the improvement in the corrosion resistance can be seen by forming the diffused layer of Zn on the surface. However, because the diffused layer of Zn formed on the surface is restricted in thickness to several .mu.m or so per side, and further that, in this case, the surface becomes a Cu-Zn alloy (so-called brass), the problem arises of Zn disappearing through the dezincificative corrosion inherent to brass. Thus, the sacrificial anode effect of Zn cannot be retained over a long period of time.
As described above, although the diffused layer of Zn formed on the surface is restricted to several .mu.m or so per side in thickness, if the dezincificative corrosion inherent to brass can be suppressed and prevented effectively, a more corrosion resistant fin material for heat-exchangers could be expected, while thinning of the fin material would also become possible.
In order to suppress such dezincificative corrosion inherent to brass, a method is conceivable wherein a third element is added into the diffused layer of Cu-Zn, in order to improve the corrosion resistance. Thus, the Zn-diffused layer would become highly corrosion-resistant.
Various elements can be considered for suppressing the dezincificative corrosion. However, generally, remarkably large decreases in the thermal conductivity occur when adding these elements to copper, compared to the same fin material which adds the same amount of Zn. Hence, if these elements are added to the entire diffused layer in a sufficient amount to suppress and prevent effectively the dezincificative corrosion etc., the corrosion resistance would be improved, but the decrease in the thermal conductivity would end up becoming large.
As a result of extensive investigations in view of this situation, a copper fin material for heat-exchangers, excellent in both corrosion resistance and thermal conductivity, and a method of producing the same have been developed. According to the present invention, the dezincificative corrosion of a Zn-diffused layer formed on the surface of a Cu or Cu alloy strip is alleviated, while the decrease in thermal conductivity arising from the addition of a third element into the Zn-diffused layer is lessened.